There's a "registration lock" feature for exactly this problem. Turn it on and you need your pin to re-register. (It expires after 7 days). Not perfect by any

There's a "registration lock" feature for exactly this problem. Turn it on and you need your pin to re-register. (It expires after 7 days). Not perfect by any means, but it seems like a well balanced compromise, given the decision to use phone numbers in the first place.